Code Runner MCP server

Code Runner MCP provides a secure sandboxed environment for executing JavaScript, TypeScript, and Python code with support for package imports. It allows you to test packages, run code for logic verification, and execute functions without compromising security, making it an excellent tool for AI-assisted coding tasks.

Installation

You can set up Code Runner MCP using either Node.js or Deno. Choose the method that works best for your environment.

Using Node.js

If you have Node.js installed, configure your MCP server with the following JSON configuration:

{
  "mcpServers": {
    "code-runner": {
      "command": "npx",
      "args": [
        "-y",
        "deno",
        "run",
        "--allow-all",
        "jsr:@mcpc/code-runner-mcp/bin"
      ],
      "env": {
        "DENO_PERMISSION_ARGS": "--allow-net"
      },
      "transportType": "stdio"
    }
  }
}

Using Deno

If you have Deno installed, use this configuration instead:

{
  "mcpServers": {
    "code-runner": {
      "command": "deno",
      "args": ["run", "--allow-all", "jsr:@mcpc/code-runner-mcp/bin"],
      "env": {
        "DENO_PERMISSION_ARGS": "--allow-net"
      },
      "transportType": "stdio"
    }
  }
}

Usage

Setting Execution Permissions

For JavaScript/TypeScript execution, you can configure the permission level using the DENO_PERMISSION_ARGS environment variable:

• By default, no execution permissions are granted
• Add permissions like network access with --allow-net
• Combine multiple permissions with spaces, e.g., --allow-env --allow-net

Checking the Environment

To verify the current execution environment versions, you can run:

// For JavaScript/TypeScript version:
console.log(`Deno version: ${Deno.version.deno}`);
console.log(`TypeScript version: ${Deno.version.typescript}`);

// For Python version (using Pyodide):
import * as pyodide from "pyodide";
await pyodide.loadPyodide();
console.log(pyodide.runPython("import sys; print(sys.version)"));

Runtime Environments

JavaScript/TypeScript Runtime

Code Runner MCP uses Deno to execute JavaScript and TypeScript code with these features:

• Native TypeScript support without compilation steps
• ES module standards and dynamic imports
• NPM package import using npm: specifier
• Strict permission policies for security

Example of importing and using packages:

// Import a package from npm
import _ from 'npm:lodash';

// Use the imported package
const result = _.chunk(['a', 'b', 'c', 'd'], 2);
console.log(result);

Python Runtime

Python code execution uses Pyodide (Python compiled to WebAssembly):

• Runs in an isolated WASM virtual machine
• Package management through micropip
• Support for many pure Python packages and scientific libraries

Example of importing and using a Python package:

import micropip
await micropip.install('numpy')

import numpy as np
data = np.array([1, 2, 3])
print(f"Mean: {data.mean()}")

Important Notes

• First-time package imports require installation time, but subsequent runs use cached packages
• Not all PyPI packages can be installed in Pyodide, especially those with C extensions
• JavaScript/TypeScript execution permissions must be explicitly granted through the configuration

How to install this MCP server

For Claude Code

To add this MCP server to Claude Code, run this command in your terminal:

claude mcp add-json "code-runner" '{"command":"npx","args":["-y","deno","run","--allow-all","jsr:@mcpc/code-runner-mcp/bin"],"env":{"DENO_PERMISSION_ARGS":"--allow-net"},"transportType":"stdio"}'

See the official Claude Code MCP documentation for more details.

For Cursor

There are two ways to add an MCP server to Cursor. The most common way is to add the server globally in the ~/.cursor/mcp.json file so that it is available in all of your projects.

If you only need the server in a single project, you can add it to the project instead by creating or adding it to the .cursor/mcp.json file.

Adding an MCP server to Cursor globally

To add a global MCP server go to Cursor Settings > Tools & Integrations and click "New MCP Server".

When you click that button the ~/.cursor/mcp.json file will be opened and you can add your server like this:

{
    "mcpServers": {
        "code-runner": {
            "command": "npx",
            "args": [
                "-y",
                "deno",
                "run",
                "--allow-all",
                "jsr:@mcpc/code-runner-mcp/bin"
            ],
            "env": {
                "DENO_PERMISSION_ARGS": "--allow-net"
            },
            "transportType": "stdio"
        }
    }
}

Adding an MCP server to a project

To add an MCP server to a project you can create a new .cursor/mcp.json file or add it to the existing one. This will look exactly the same as the global MCP server example above.

How to use the MCP server

Once the server is installed, you might need to head back to Settings > MCP and click the refresh button.

The Cursor agent will then be able to see the available tools the added MCP server has available and will call them when it needs to.

You can also explicitly ask the agent to use the tool by mentioning the tool name and describing what the function does.

For Claude Desktop

To add this MCP server to Claude Desktop:

1. Find your configuration file:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json
• Windows: %APPDATA%\Claude\claude_desktop_config.json
• Linux: ~/.config/Claude/claude_desktop_config.json

2. Add this to your configuration file:

{
    "mcpServers": {
        "code-runner": {
            "command": "npx",
            "args": [
                "-y",
                "deno",
                "run",
                "--allow-all",
                "jsr:@mcpc/code-runner-mcp/bin"
            ],
            "env": {
                "DENO_PERMISSION_ARGS": "--allow-net"
            },
            "transportType": "stdio"
        }
    }
}

3. Restart Claude Desktop for the changes to take effect